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Since 2005, the world’s first free-electron laser in the X-ray range, FLASH at DESY in Hamburg, has been generating a very special kind of light for researchers from all over the world: extremely intense, ultrashort pulsed X-ray laser flashes. Researchers are using them to observe the movements of atoms and molecules.

FLASH accelerates electrons to almost the speed of light. Special arrangements of magnets, called “undulators”, force the electrons to move along a slalom course. In the curves, the fast particles emit short flashes of X-ray light. These flashes overlap and oscillate in sync to create incredibly intense and short light flashes on a timescale of femtoseconds – in other words, quadrillionths of a second. This is so interesting because chemical reactions also take place at femtosecond speeds. FLASH is the pioneering light sourceat which important foundations for films of the nanocosmos have been investigated.

Filming chemical reactions

To observe the details of a chemical reaction, researchers first take a huge number of individual images until they have recorded all of the stages of the reaction. Arranging these images in sequence creates a “film” that shows the process at the molecular level. With X-ray lasers such as FLASH, scientists can observe what actually happens at the level of atoms and molecules and use this information to develop new materials and drugs

The international research community is tremendously interested in FLASH. That is why the facility was expanded and a second tunnel for undulators was built branching off from the accelerator tunnel. The X-ray flashes from this tunnel are guided into a new experimental hall with several measuring stations that offer plenty of space for additional experiments.

TESLA accelerator technology

A distinguishing feature of FLASH is the use of superconducting accelerator technology to propel the electrons to the required high energy. This pioneering technology was developed by DESY together with 50 institutes from 12 countries within the international TESLA Technology Collaboration. Unlike conventional facilities, the TESLA accelerator elements – the resonators – are superconducting: cooled to minus 271 degrees Celsius, they conduct electric current loss-free, so that practically all of the electric power they consume can be transferred to the particles – an extremely efficient acceleration method. What’s more, the superconducting resonators deliver a very thin and homogeneous electron beam of extremely high quality. A particle beam with such special properties is a prerequisite to operate a free-electron laser in the X-ray region.

Free-electron lasers – the principle

In a free-electron laser, the special X-ray laser light is produced based on a sophisticated principle: During their slalom run through a periodic array of magnets (the undulator), the electron bunches emit radiation (photons) of a distinct wavelength. The photon beam propagates in a straight line so that it overlaps with the electron bunch. It imprints its periodic structure on the electron bunch, so that the initially homogeneous charge density distribution becomes periodic – a chain of tiny individual charge “disks” regularly separated by a single wavelength. Now all the electron disks emit radiation in synchronism, and the light can amplify itself to form high-intensity laser radiation.